THE SEGMENTATION OF THE HEAD. 139 



Two years later (1871) Gegenbaur attacked the same problem, 

 and between his results and those of Huxley there is much 

 similarity. With him the cranial nerves are the primary test, 

 and he tries to bring the gill clefts into accordance with them. 



Of the cranial nerves modern human anatomists recognize 

 twelve pairs, known by name and number as follows : 



I. Olfactory. VII. Facial. 



II. Optic. VIII. Auditory. 



III. Oculomotor IX. Glossopharyngeal. 



IV. Trochlearis. X. Vagus (Pneumogastric). 

 V. Trigeminal. XI. Spinal Accessory. 



VI. Abducens. XII. Hypoglo^sal. 



Like Huxley, Gegenbaur at once threw three of these out of 

 discussion, for it was thought that the nerves of special sense- 

 olfactory, optic, auditory differed from the rest in their mode 

 of growth, were, in fact, outgrowths of the brain proper. 

 Others, from the nature of their functions, were relegated to a 

 secondary position. As long ago as 1807, Sir Charles Bell had 

 pointed out that the nerves of the spinal cord had two roots, 

 the dorsal one with an enlargement, or ganglion, the ventral 

 non-ganglionated, and that these roots differed entirely in their 

 function. Through the dorsal root, sensations were brought 

 from the peripheral parts to the brain, while through the ven- 

 tral root the actions of the muscles and other structures were 

 controlled. Hence, since his time, these roots have been called 

 respectively sensory and motor. In the brain-region, however, 

 this distinction of roots is not so clearly shown. Some of the 

 nerves,- like the third, fourth, and sixth, have purely motor 

 functions, and these motor nerves were consequently relegated 

 to a secondary position ; dorsal or sensory roots were taken as 

 a test. 



Now, applying these ideas, Gegenbaur concluded that since 

 the vagus nerve was distributed to several gill clefts, it must 

 be regarded as a composite nerve, while the glossopharyngeal, 

 supplying a single cleft, was simple, as also was the facial. 

 The trigeminal, however, had too many branches to supply a 

 single cleft, and so this was regarded as a double nerve. 



